In an LTE (Long Term Evolution) being one standard of a radio communication system regulated by 3GPP (3rd Generation Partnership Project), a radio terminal (User Equipment: UE) selects one cell formed by a downlink (Downlink: DL) component carrier (hereinafter, referred to as a DL carrier) and an uplink (Uplink: UL) component carrier (hereinafter, referred to as a UL carrier) associated with the DL carrier, and performs communication with a radio base station (evolved Node B: eNB) in the cell. Further, a selection of the cell of the radio terminal (UE) basically corresponds to that of the DL carrier. In the LTE, by using the DL carrier, the radio base station (eNB) periodically transmits a reference signal (Cell specific Reference Signal: CRS), a synchronization signal (Synchronization Signal: SS), system information (System Information: SI), and the like that are mutually used by the radio terminals (UEs) within the cell. In the LTE, the above signals and information are signals and information needed when the radio terminal (UE) selects the cell. Further, a transmission configuration (for example, a transmission period or a signal arrangement) is regulated so as to satisfy predetermined conditions.
A cell selection method and a cell reselection method of the radio terminal (UE) in the idle state (Radio Resource Control (RRC) Idle) in the LTE will be described with reference to FIG. 12. In the cell selection of the radio terminal (UE) in the idle state (RRC_Ldle), in accordance with a capability of a radio frequency unit, the radio terminal (UE) searches (scans) in all frequencies in a frequency band of E-UTRA (Evolved-Universal Terrestrial Radio Access) supported by the radio terminal (UE) itself.
In FIG. 12, when having a capability of detecting the cell by using a frequency 1 (f1), a frequency 2 (f2), and a frequency 3 (f3), the radio terminal (UE) tries to detect a suitable cell by using all of the f1 to f3. In each frequency, the radio terminal (UE) may search a strongest cell (for example, a cell in which a detection level of CRS is highest, received power of CRS (RSRP) is largest, or received quality of CRS (RSRQ) is highest). On the other hand, when the radio terminal (UE) has information about cell parameters or information about frequency indicated by previously-received measurement control information, the above information may be used. In this case, the entire frequency band of E-UTRA supported by the radio terminal (UE) need not be searched.
When a suitable cell is found out, the radio terminal (UE) selects the cell and camps on the cell. FIG. 12 illustrates a situation in which the radio terminal (UE) selects a Cell 2. Here, the suitable cell is a cell that satisfies predetermined cell selection criteria, and on which the radio terminal (UE) is permitted to camp (and access the radio base station (eNB)). As the predetermined cell selection criteria, it is specified that minimum received power or minimum received quality necessary for camping is satisfied (Non-Patent Literature 1). A start of search from which frequency band depends on policies of designing of the radio terminal (UE). Further, information about a frequency band in the cell on which the radio terminal (UE) previously camps may be stored and the search may be started from the frequency band.
The cell reselection method of the radio terminal (UE) in the idle state (RRC_Ldle) will be described.
First, the cell reselection to a cell of the same frequency is started from ranking for any cell(s) that satisfy the cell selection criteria. When an offset value of neighbouring cells to the serving cell is indicated by using broadcast information from the radio base station (eNB), the radio terminal (UE) performs ranking in consideration of the offset value. Further, the radio terminal (UE) selects the cell that satisfies predetermined first cell reselection criteria for the cell ranked as a best cell (best cell). As the first cell reselection criteria, it is specified that one or more seconds have elapsed since the radio terminal (UE) camped on the current serving cell and a target cell (a candidate of a new serving cell) is ranked upper than the serving cell for a predetermined period. When the cell that satisfies the criteria is not present, or when the serving cell is ranked as the best cell, the cell reselection is not performed. In order that an unnecessary measurement may be prevented from being performed, when received power or received quality of the cell (serving cell) during the camping becomes smaller than a predetermined threshold, the radio terminal (UE) may perform the measurement.
Next, control of the cell reselection with respect to a cell of the frequency different from that of the serving cell or control of the cell reselection with respect to a cell of different radio access technology (Radio Access Technology: RAT) is performed in accordance with any one of the following items.
1) Cell reselection to neighbouring cell of the LTE or another RAT, having higher priority:
When a frequency that is higher than the frequency of the serving cell and to which a cell reselection priority is allocated is present, the radio terminal (UE) performs measurement (Inter-frequency measurement) of the cell in the LTE of the frequency, or performs measurement (Inter-RAT measurement) of the cell of another RAT. Further, the radio terminal (UE) selects the cell that satisfies predetermined second cell reselection criteria. As the second cell reselection criteria, it is specified that one or more seconds have elapsed since the radio terminal (UE) camped on the current serving cell, and a situation in which received power or received quality of the cell is larger than respective decision threshold has been continued for a predetermined period.
2) Cell reselection to neighbouring LTE cell having the same priority:
The measurement (Inter-frequency measurement) of the cell in the LTE of a frequency allocated the same cell reselection priority as the frequency of the serving cell is performed, and the cell that satisfies the predetermined third cell reselection criteria is selected. The third cell reselection criteria are the same as the above-described cell selection criteria, and a candidate cell for the cell reselection is a cell in the LTE of different frequency.
3) Cell reselection to neighbouring cell of the LTE or another RAT, having lower priority:
The measurement (Inter-frequency measurement) of the cell in the LTE of a frequency allocated a lower cell reselection priority than the frequency of the serving cell, or the measurement (Inter-RAT measurement) of the cell of another RAT of a frequency allocated a lower cell reselection priority than the frequency of the serving cell is performed, and the cell that satisfies the predetermined fourth cell reselection criteria is selected. As the fourth cell reselection criteria, it is specified that one or more seconds have elapsed since the radio terminal (UE) camped on the current serving cell, and a situation in which received power or received quality of the serving cell is smaller than a decision threshold and received power or received quality of neighbouring cells is larger than another decision threshold has been continued for a predetermined period.
Further, the cell reselection priority is broadcast by using system information (System Information), or is informed by using a radio terminal (UE) individual signal (Dedicated Signaling).
Through the control as described above, the radio terminal (UE) in the idle state (RRC_Idle) is capable of performing a suitable cell selection and cell reselection.
On the other hand, in 3GPP, LTE-Advanced, where the LTE is enhanced and a function is largely extended, is studied. In the LTE, in addition to the reference signal (Cell specific Reference Signal: CRS), the radio base station (eNB) transmits also a reference signal (Demodulation RS: DM-RS) for demodulating user data, a reference signal (CSI-RS) for measuring or calculating a communication path state information (Channel State Information: CSI) of a downlink, a downlink physical control channel (Physical Downlink Control Channel: PDCCH) including scheduling information, and the like. In a study of the LTE-Advanced, it is pointed out that an overhead of the above-described signals or information other than downlink user data is large and usage of downlink radio resources is not optimized. For the purpose of improving downlink throughput or cell capacity, the overhead is studied to be reduced. For example, a method for reducing the number of symbols of the reference signal (CRS) for transmission per sub-frame (Subframe), reducing the number of sub-frames (Subframes) for transmitting the reference signal (CRS), or not absolutely transmitting the reference signal (CRS) is studied. Further, a type of the above DL carrier is called a New Carrier Type (NCT) (Non-patent literature 2). Hereinafter, with respect to a conventional component carrier (Legacy Component Carrier: LCC), the component carrier of the NCT is called a New Type Component Carrier (NTCC). The NTCC is supposed to be used as a secondary component carrier (Secondary Component Carrier: SCC) (also called a Secondary Cell (SCell)) of a carrier aggregation (Carrier Aggregation: CA) by which the radio terminal (UE) uses a plurality of component carriers at the same time and performs communication with the radio base station (eNB). Here, a component carrier (cell) in which the radio terminal (UE) establishes a connection to the radio base station (eNB) and acquires basic information such as security information is called a primary component carrier (Primary Component Carrier: PCC)/primary cell (Primary Cell: PCell). Further, an additional component carrier/cell that is used together with the primary component carrier/primary cell (PCC/PCell) is called the secondary component carrier/secondary cell (SCC/SCell). It can be expected that radio resources for transmitting downlink user data are increased in the secondary component carrier/secondary cell (SCC/SCell) and downlink throughput or cell capacity is improved through an introduction of DL NTCC. Further, power saving of a radio network can be also expected.